摘要 |
An optical time domain reflectometry method of sensing a parameter to be measured in a region of interest comprises launching optical radiation ( 4 ) at a probe wavelength into an optical fibre ( 5, 6 ) and producing electrical output signals in response to optical radiation backscattered from the optical fibre ( 5, 6 ). In one aspect of the invention the optical fibre comprises first and second sections ( 5, 6 ), the second section ( 6 ) having a lower intensity threshold for the onset of non-linear effects than the first section ( 5 ) and being deployed in the region of interest. Optical radiation ( 4 ) at the probe wavelength is launched into the said first section ( 5 ) at an intensity lower than the non-linear effects intensity threshold of the first section ( 5 ) but higher than the non-linear effects intensity threshold of the second section ( 6 ), the attenuation characteristics of the said first section ( 5 ) being chosen so that the intensity of the optical radiation ( 4 ) at the probe wavelength reaching the second section ( 6 ) is below that of the non-linear effects intensity threshold of the said second section ( 6 ). In another aspect of the invention the optical fibre comprises first and second sections connected together by a remote amplifier, the gain of the amplifier being selected so as to compensate for attenuation losses in the probe wavelength in the first section.
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